Is there an easy way to tell the Mail app (OS X 10.8) to take my signature from a file ? I can manually edit it in the config, but I have a rotating signature and the signature file is updated automatically, so I'd like Mail to use that. Many mail programs allow the signature to be taken from a local file, but there doesn't seem to be a way to do this for Mail.

p.s I'm aware that I could dig into the configuration subdirectory, but the information stored there is an HTML-ized version of the signature, and while I could probably write a script that converts my file into the desired format and then copies it over there, it's a pain because it's not compatible with the signature files on my other systems (I use mac/windows/linux systems in different settings)